This study has been designed to (i) define the parameters which best represent the histological and ultrastructural characteristics of naturally occurring root caries lesions (Phase I), (ii) to develop an in vitro model which simulates, as judged by the parameters defined in Phase I, in vivo root caries (Phase II). The methods by which the parameters will be defined include transmitted light microscopy, scanning electron microscopy, microradiography and polarized light microscopy. In Phase I teeth exhibiting a gradation of naturally occurring root caries lesions will be serially sectioned and each section will be examined by the above techniques. The results from these investigations will be correlated and used to define the characteristics of the lesions both qualitatively and semi-quantitatively. The in vitro models to be tested are a purely chemical acid gel system acidified with lactic acid and a bacterial acid gel system using Streptococcus mutans and Actinomyces viscosis. Varnished teeth with windows left exposed on enamel, at the crown/root interface and on the root surface, are placed into the two systems. Teeth are removed at certain intervals and serially sectioned before examination by the four methods mentioned earlier. These in virto lesions will then be compared with the base-line data established in Phase I. At the end of Phase II it should be possible to define the gel concentrations and teeth exposure times which will simulate the range of root caries seen in vivo. The future aims of this study are to use the in votro system to: (1) idnetify the underlying mechanisms associated with the process of root caries; (2) determine the resistance of the various component tissues to caries; (3) to investigate the theories of site susceptibility to caries; (4) test preventive agents or restorative materials which may, in fact, serve to test the proposed mechanisms and (5) ultimately to use clinically the knowledge gained to retard, reverse and prevent root caries.